Although not yet mass produced, internal combustion engines having injection of gasoline directly into the combustion chamber and auto-ignition as in the diesel principle are known. This operating mode is referred to below as “homogeneous auto-ignition.” Additional savings in fuel consumption are expected with this homogeneous auto-ignition operating mode of gasoline engines, and emission results should be even better than with internal combustion engines having direct gasoline injection and external ignition by a spark plug.
Homogeneous auto-ignition is achieved in a gasoline engine by virtue of the fact that a considerable portion of the combusted air-fuel mixture is not expelled into the exhaust but instead remains in the combustion chamber (so-called internal exhaust gas recirculation). The combusted air-fuel mixture is referred to below as residual gas (RG). With internal exhaust gas recirculation, either residual gas is retained in the combustion chamber (negative valve overlap) or residual gas is withdrawn back out of the exhaust channel or the intake channel (positive valve overlap) through variable triggering of the intake and exhaust valves at top dead center in the exhaust and refill charge cycle.
In the subsequent intake stroke, the residual gas is mixed with combustion air and fuel. Therefore, at the time of closing the intake valve(s), the gas mixture of combustion air and residual gas in the combustion chamber is at a much higher temperature than in normal operation. Compression of the gas mixture in the combustion chamber after the intake stroke causes the temperature to rise to the extent that the fuel-air mixture in the combustion chamber ignites spontaneously without external ignition by a spark plug. The goal of the homogeneous auto-ignition operating mode is that auto-ignition of the fuel-air mixture in the combustion chamber will occur approximately on reaching the ignition top dead center.
The residual gas has two important functions. First, the hot residual gas supplies heat, which, in combination with the rise in temperature in the compression stroke, allows auto-ignition of the fuel-air mixture.
The second function of the residual gas is to retard the kinetics of combustion triggered by auto-ignition to thereby reduce the mechanical load on the internal combustion engine, reduce noise production and prevent local temperature peaks from developing. Therefore, engine efficiency is improved as a result and operation of the internal combustion engine with little or even no NOx is possible because of the lower maximum temperatures.
These functions of the residual gas remaining in the combustion chamber at the same time result in restrictions on homogeneous auto-ignition operating mode to speeds amounting to approximately 50% of the maximum speed of the internal combustion engine. Furthermore, homogeneous auto-ignition operating mode may take place only up to a load of approximately ⅓ of full load of a naturally aspirated internal combustion engine. If the load is further increased, the reaction kinetics is greatly accelerated because of the reduced amount of residual gas, generating unwanted noise and resulting in a very high mechanical load on the internal combustion engine.
Another restriction on the range of use of homogenous auto-ignition operating mode occurs at speeds below 2000 rpm and at very low loads of 10% to 15% of full load, for example, because adequate cylinder filling with combustion air is impossible below this speed and at loads below 15%, and therefore this mode is not usable.
An object of the present invention is to provide a method for controlling an internal combustion engine and to provide an internal combustion engine which makes it possible to expand the characteristics map range within which the internal combustion engine is operated with homogeneous auto-ignition and therefore to further improve the emission and consumption performance of naturally aspirated internal combustion engines.